Payment to Avoid Garnishment - Ohio

Hi... I have a $5800 loan that I am currently paying $222.61 semi-monthly to a collection agency as payment to avoid garnishment. Typically this time of year, I change my IRS withholdings on my paycheck to get a little extra hoiday money for presents. I was looking at the form that they sent me and even though my take home income would increase if I change this, the amount of my payment to them would actually decrease based on the formula's on that form. I don't understand why it's set up that way, but it would result in me paying them almost $75 less a paycheck. I would actually prefer to give them a lesser amount just because they've been a pain since I started making payments. And for once, it'd be nice to stick it to them since I had actually paid them $600 extra on my account over the last 9 months and they still proceeded this route. So, if I have the opportunity to legally stick it to them some without taking the risk of employer garnishment (I'll get fired for garnishments), it'd be nice bonus for me. :)

My question is, once I signed that original "Payment to Avoid Garnishment" and started making payments to them, can I sign a new form and mail it to them with the lowered payment because my income changed? Or do I have to go to court now to try to get that amount lowered.

I'll also mention that the agency is Weltman, Weinburg and Reis which I know are very difficult to deal with.

Any information would be greatly appreciated. :confused: